Create an R-level Task Callback Manager

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Description

This provides an entirely S-language mechanism
for managing callbacks or actions that are invoked at
the conclusion of each top-level task. Essentially,
we register a single R function from this manager
with the underlying, native
task-callback mechanism and this function handles invoking the other
R callbacks under the control of the manager.
The manager consists of a collection of functions that access shared
variables to manage the list of user-level callbacks.

Usage

Arguments

handlers

this can be a list of callbacks in which each element
is a list with an element named "f"
which is a callback function, and an optional
element named "data" which is the 5-th argument to be
supplied to the callback when it is invoked.
Typically this argument is not specified, and one uses add to
register callbacks after the manager is created.

registered

a logical value indicating whether
the evaluate function has already been registered
with the internal task callback mechanism.
This is usually FALSE and
the first time a callback is added
via the add function, the
evaluate function is automatically registered.
One can control when the function is registered
by specifying TRUE for this argument
and calling addTaskCallback manually.

verbose

a logical value, which if TRUE,
causes information to be printed to the console
about certain activities this dispatch manager performs.
This is useful for debugging callbacks and the handler
itself.

Value

A list containing 6 functions:

add

register a callback with this manager, giving the
function, an optional 5-th argument, an optional name
by which the callback is stored in the list,
and a register argument which controls whether
the evaluate function is registered with the internal
C-level dispatch mechanism if necessary.

remove

remove an element from the manager's collection
of callbacks, either by name or position/index.

evaluate

the ‘real’ callback function that is registered
with the C-level dispatch mechanism and which invokes each of the
R-level callbacks within this manager's control.

suspend

a function to set the suspend state
of the manager. If it is suspended, none of the callbacks will be
invoked when a task is completed. One sets the state by specifying
a logical value for the status argument.

register

a function to register the evaluate
function with the internal C-level dispatch mechanism. This is
done automatically by the add function, but can be called
manually.